Porous intermediate layer for affixing lossy coatings to r.f. tube circuits



1969 M. FEINLEIB' 3,475,707

POROUS INTERMEDIATE LAYER FOR AFFIXING LOSSY COATINGS TO R.F. TUBE CIRCUITS Filed Dec. 21, 1966 PARTS FORMED PARTS CLEANED PARTS com-:0

WITH POWDERED METAL I FIG. 3 I

PARTS HEATED T0 SINTER AL FIG'4 MET POWDER PARTS COATED WITH RESISTIVE MATERIAL A PARTS BRAZED TOGETHER TO FORM THE R.F. CIRCUIT INVENTOR.

MORRIS FEINLEIB ATTORNEY United States Patent 3,475,707 POROUS INTERMEDIATE LAYER FOR AFFIXING LOSSY COATINGS TO R.F. TUBE CIRCUITS Morris Feinleib, Los Altos, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Dec. 21, 1966, Ser. No. 603,569 Int. Cl. H01p 7/06 US. Cl. 333-83 6 Claims ABSTRACT OF THE DISCLOSURE Description of the prior art Heretofore, lossy coatings such as Kanthal coatings have been applied to R.F. circuits such as to the inside walls of coupled cavity slow wave circuits used in high power microwave tubes. The lossy coating is formed on such circuits by first sandblasting the surfaces of the cavity parts which are to be coated with the lossy material, typically, a material comprising by weight 5.5% aluminum, 22% chromium, 0.5% cobalt, and the balance iron and marketed under the trade name Kanthal A-l by The Kanthal Corporation of Bethel, Conn.

The sandblasted parts are typically deformed by the sandblasting and, thus, have to be straightened. Not all of the parts can be properly straightened, thereby resulting in a substantial amount of scrap. The sandblasted and straightened parts are then flame spray coated with Kanthal A-l. Once coated, the parts are assembled and brazed together to form the composite slow wave circuit. During the brazing operation and in subsequent use, it is found that the lossy coating does not always adhere properly to the circuit producing undesired flaking and peeling of the lossy coating. Once the tube has been assembled and processed, such flaking and peeling can cause destruction of the tube.

Summary of the invention In the present invention, it has been found that such aforementioned flaking and peeling of the lossy coating can be eliminated if the metallic surfaces of the R.F. circuit, which are to be coated with the lossy material, are roughened by sintering a layer of metallic particles to the circuit instead of sandblasting these surfaces.

The sintered metallic layer provides a porous substrate which is compatible with the metallic circuit and which does not peel or flake. The lossy coating is then applied over the porous substrate as by flame spraying, painting, etc. The lossy coating infiltrates the pores of the porous substrate to provide a keying function which prevents flaking or peeling of the lossy coating from the circuit. Another advantage is that the lossy coating is affixed to the circuit without having to perform the sandblasting and straightening operations thereby eliminating distortion of the circuit parts and the scrap that resulted therefrom. Moreover, the resultant lossy coating adheres to the circuit better than previous coatings.

The principal object of the present invention is the provision of R.F. circuits having improved lossy coatings and tubes using same.

One feature of the present invention is the provision of a layer of porous metal formed on an R.F. circuit ice with a lossy coating formed over the porous metal substrate, whereby the lossy coating infiltrates the pores of the porous substrate for holding the lossy coating to the R.F. circuit.

Another feature of the present invention is the same as the preceding feature wherein the porous metallic layer is formed of sintered metal particles also sintered to the R.F. circuit.

Another feature of the present invention is the same as the preceding feature wherein the met-a1 particles are selected from the class consisting of copper, nickel and iron.

Another feature of the present invention is the same as the preceding feature wherein a preponderance of the particles have a size falling within the range of 10 to microns in diameter.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

Brief description of the drawings FIG. 1 is a schematic longitudinal sectional view of a microwave tube incorporating features of the present invention,

FIG. 2 is an exploded perspective view of a cavity portion of the slow wave circuit of FIG. 1 delineated by line 22,

FIG. 3 is a flow diagram, in block form, depicting the method of the present invention,

FIG. 4 is an enlarged fragmentary sectional view of a portion of the structure of FIG. 2 delineated by line 4-4, and

FIG. 5 is a perspective view of an assembled portion of the slow wave circuit of FIG. 1.

Description of the preferred embodiment Referring now to FIG. 1, there is shown a microwave tube 1 having a radio frequency slow wave circuit incorporting features of the present invention. The tube 1 includes an electron gun assembly 2 for forming and projecting a beam of electrons 3 over a predetermined beam path to a beam collector assembly 4. A plurality of cavity resonators 5, 6 and 7 are arranged along the beam 3 for electromagnetic interaction therewith. A slow wave circuit 8 is disposed along the beam 3 between cavity resonator 7 and the collector 4. An electrical solenoid 9 surrounds the beam 3, cavities, and slow wave circuit 8 for focusing the beam 3 from the gun 2 to the beam col lector 4.

In operation, the input cavity resonator 5 is excited with R.F. Wave energy, at a frequency to be amplified, via input coaxial line coupler 11. The electric fields of the input cavity 5 velocity modulate the beam with the signal energy. The modulated beam produces excitation and further modulation of the beam by the successive buncher cavities 6 and '7. The modulated beam excites a signal wave on the slow wave circuit 8. The signal wave grows in amplitude on the slow wave circuit with further cumulative electronic interaction with the beam 3. A greatly amplified signal wave is extracted from the slow wave circuit 8 and fed to a suitable utilization device, not shown, via output waveguide 12 and window assembly 13.

A suitable slow wave circuit 8 is a cloverleaf slow wave circuit formed by a plurality of inductively coupled cavity resonators 14. Each cavity resonator 14 is formed by a cloverleaf shaped side wall 15 (see FIG. 2) closed at its ends by transverse walls 16. The transverse walls 16 are common wall structures for adjacent coupled cavities 14 and each includes a central beam hole 17 and eight radially directed inductive coupling slots 18 equally angul-arly spaced about the circumference of the end wall 16. Adjacent cavity resonators 14 are angularly displaced with respect to each other by 45 in order to provide negative mutual inductive coupling through the slots 18 and to provide a fundamental space harmonic forward wave slow wave circuit having reltively high power handling capabilities.

In order to suppress unwanted oscillations in the slo wave circuit, when operating at high power levels, the last 6 to 8 cavity resonators 14 are coated on their interior surfaces with a resistive (lossy) material such as, for example, the aforementioned Kanthal A-l, The resistive material serves to absorb radio frequency wave energy on the circuit 8, thereby heavily loading certain of the possible interfering modes of oscillation and thus, preventing these modes from storing sufiicient energy to produce regenerative oscillation.

Referring now to FIG. 3, there is shown a flow diagram depicting the steps in the method of the present invention for producing the slow wave circuit 8 with the resistive coating thereon. More specifically, the parts 15 and 16 are formed of a suitable thermally and electrically conductive material such as copper. Once formed the end and side wall parts 15 and 16, which are to be coated with the resistive material, are cleaned to remove grease and other surface contaminants. A suitable cleaning is provided by standard degreasing operations such as, for example, vapor degreasing in trichlorethylene or washing the parts with acetone.

After the parts are cleaned and dry they are coated with a layer of powdered metal particles with a preponderance of such particles falling within the size range of microns to 150 microns in diameter, and preferably 25 to 150 microns in diameter. In a preferred embodiment, the metal particles are of the same material as that of the material on which they are to be coated. Thus, for a copper radio frequency circuit, the particles are preferably copper. However, other metal particles may also be used. For example, iron or nickel particles may be employed. The particles need not be pure metal but may be oxides or certain other compounds of the metals. For example, powders of cuprous oxide, cupric oxide, nickel oxide, nickel-phosphorus alloy, iron oxide, etc. may be employed either instead of or in conjunction with the metal powders.

The powdered metal is preferably applied as a metal paint which may be sprayed or brushed onto the parts. A suitable metal paint for spraying the parts 16 and 17 is as follows: 3000 grams of metal powder such as Fernlock copper powder type D100 as made by Malone Metal Powders Inc., of Flemington, NJ., is mixed with 300 grams of a binder. A suitable binder is 25% solution of ethyl cellulose, grade N-22, in a solvent formed by 1 part by weight of methanol to 3 parts toluene. The metal powder and binder are then mixed with 1900 grams of amyl acetate to form the paint. The metal paint is then sprayed, brushed or otherwise applied to the parts and 16 and allowed to dry to form a coating of metal particles.

The coated parts 15 and 16 are then heated in a reducing atmosphere to a temperature slightly below the melting point of the powder and parts 15 and 16 in order to sinter the powder coating together and to the parts 15 and 16. In the case of copper powder on copper parts 15 and 16, the parts are typically heated to 1000 C. for 15 minutes. A suitable reducing atmosphere is moist or any hydrogen gas or forming gas or dissociated ammonia gas. When the metal particles are sintered together they form a porous metal layer 21 (see FIG. 4) firmly adhering to the metal R.F. circuit parts 15 and 16.

After the porous metal layer 21 has been formed on the parts 15 and 16 they are coated with a layer of resistive material 22 such as Kanthal A1, Ni, Kovar, iron, carbon or tungsten carbide. Such resistive materials may be applied by painting, flame spraying or plasma gun spraying. When applied, the resistive material infiltrates at least the surface pores of the porous metal layer 21. In this manner, the porous metal layer performs a keying type function for keying and fixedly adhering the resistive layer 22 to the porous metal layer 21 and hence, to the radio frequency circuit parts 15 and 16.

In a preferred embodiment, Kanthal A-l is flamesprayed onto a porous copper layer 21. In this case, the resistive material is molten when it contacts the porous surface and the molten Kanthal infiltrates the surface 10 pores of the copper layer 21. The Kanthal is normally applied to a thickness of 0.001" to 0.005.

After the parts 15 and 16 have been coated with the resistive layer 22 they are brazed together to form the composite coupled cavity radio frequency slow wave circuit 8, as partially shown in FIG. 5. It has been found that the adherence of Kanthal to copper radio frequency circuits is greatly enhanced by the provision of the porous copper layer 21.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention can be made without departing from the scope thereof it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and 25 not in a limiting sense.

What is claimed is:

1. In a radio frequency tube, means forming a metallic radio frequency circuit, means forming a layer of resistive material on said radio frequency circuit for absorbing radio frequency energy on said circuit, the improvement comprising: means forming a porous metal layer on said circuit between said radio frequency circuit and said resistive layer, said resistive layer being formed directly over said porous metal layer with said resistive material being infiltrated into the pores of said porous metal layer, whereby said porous metal layer serves to fixedly secure said resistive material to said radio frequency circuit.

2. The apparatus of claim 1 wherein said porous metal to said radio frequency circuit.

3. The apparatus of claim 2 wherein a preponderance of the metal particles making up said porous metal layer have sizes falling within the range of 10 microns to 150 microns in diameter.

4. The apparatus of claim 1 wherein said porous metal layer is made of a material selected from the class consisting of copper, nickel and iron.

5. The apparatus of claim 1 wherein said resistive material is selected from the class consisting of aluminum, chromium, cobalt, iron, nickel, tungsten carbide, and carbon used singly or in alloy form.

6. The apparatus of claim 1 wherein said radio frequency circuit is made of copper, said porous metal layer is made of a material selected from the class consisting of copper, nickel and iron, and said resistive material is selected from the class consisting of aluminum, chromium, cobalt, iron, nickel carbon, and tungsten carbide, used singly or in alloy form.

References Cited UNITED STATES PATENTS 3,374,389 3/1968 'Cahour et a1. 315-35 2,880,120 3/1959 Pelle 3153.5 X

2,880,355 3/1959 Epsztein 3153.5

2,925,645 2/1960 Bell et al. 29-625 X 3,360,103 2/1968 Thall 3153.5

HERMAN KARL SAALBACH, Primary Examiner SAXFIELD CHATMON, JR., Assistant Examiner US. Cl. X.R.

layer is formed of metal particles adhered together and 

